Spray cooling system

ABSTRACT

A spray cooling system for semiconductor devices. An ink-jet type spray device sprays droplets of a cooling fluid onto the semiconductor devices. The devices vaporize the liquid, which gets passed through a roil bond panel, or other heat exchanger, and is pumped into a spring loaded reservoir that feeds the spray device.

BACKGROUND OF THE INVENTION

This invention relates generally to cooling systems for heat-generatingdevices and, more particularly, to a spray cooling system and a methodof using the spray cooling system to cool a heat source.

With the advent of semiconductor devices having increasingly largecomponent densities, the removal of heat generated by the devices hasbecome an increasingly challenging technical issue. Furthermore, typicalprocessor boards can, in some instances, include multiple CPU modules,application-specific integrated circuits (ICs), and static random accessmemory (SRAM), as well as a dc—dc converter. Heat sinks can be used toincrease the heat-dissipating surface area of such devices. However,heat sinks, and their interfaces to the cooled devices, can provideinterference in the heat flow, and can lead to uneven cooling.

Known cooling methods for semiconductors include free-flowing andforced-air convection, free-flowing and forced-liquid convection, poolboiling (i.e., boiling a liquid cooling fluid off of a submergeddevice), and spray cooling (i.e., boiling a liquid cooling fluid off ofa device being sprayed with the liquid). Because liquids typically havea high latent heat of vaporization, these latter two methods provide fora high heat-transfer efficiency, absorbing a large quantity of heat at aconstant temperature. Typically, the cooling fluid used has a relativelylow boiling point (the temperature to maintain) and is inert to the heatsource. For semiconductor devices, FED. CIR.-72, i.e., Fluorinert, soldby 3M Corporation, is one of a number of known suitable cooling liquids.

The use of these boiling/vaporizing methods is limited to a maximumpower density, the critical heat flux (CHF). At higher densities, thevaporized cooling fluid forms a vapor barrier insulating the device fromthe liquid cooling fluid, thus allowing the wall temperature of thedevice to increase greatly. This phenomenon is referred to as pooling.When a coolant is properly sprayed, it can disperse such a vapor layer,and its CHF can be well over an order of magnitude higher than the CHFof a pool boiling system. This high CHF is reliant on having a uniformspray. Thus, spray cooling presently provides the most efficient coolingfor a heat-generating device, such as a semiconductor device.

Typically, current sprayer designs employ either pressurized liquidspraying or pressurized gas atomizing. A number of factors affect theperformance of spray cooling, thus affecting the heat transfercoefficient h and/or the CHF. It is commonly understood that surfaceroughness and wettability of the sprayed component are two of thesefactors, and the orientation of the surface being sprayed can be athird. In particular, it is believed that h is higher for rough surfaceswhen using a pressurized liquid spray, and for smooth surfaces whenusing gas atomizing. Surfaces with decreased wettability appear to havea marginal increase in h.

Critical to consistent, controlled cooling is the controlled applicationof the liquid cooling fluid in a desired distribution, flow rate, andvelocity. For example, at a low mass flow rate, CHF and h increase withthe mass flow rate. However, at a critical mass flow rate, theadvantages of increased mass flow are diminished due to pooling and/ordue to a transition to single phase heat transfer. Thus, a spray coolingsystem is preferably operated uniformly at a mass flow rate defined at apoint before the critical mass flow rate is reached. All of thesefactors make critical the design of the sprayer, i.e., the design of thenozzle and its related spray devices.

Also important to the cooling system design is its operatingtemperature. In particular, it is desirable to configure the system tooperate at a high h, which will occur with a design temperature abovethe boiling temperature and below a temperature that will dry out thesprayed coolant. The amount of heat to be dissipated must be less thanthe CHF.

For pressure-assisted spraying, consistent, controlled spraying requiresone or more high pressure pumps that provide a precise pressure to pumpthe liquid through a nozzle, even at varying flow rates. Both thedistribution and the flow rate of the sprayed liquid can change withvariations in the driving pressure and/or small variations in the nozzleconstruction. Thus, the cooling system is a sensitive and potentiallyexpensive device that can be a challenge to control.

For gas atomizing, consistent, controlled spraying requires apressurized gas that is delivered to a sprayhead design in a precisemanner. Because the gas must be pressurized separately from the coolingfluid, such systems are not typically closed systems. The gas must bebled out for the condenser to run efficiently. Furthermore, both thedistribution and the flow rate of the cooling fluid can change withvariations in the gas pressure. Thus, the cooling system is a sensitiveand potentially expensive device that can be a challenge to control.

Accordingly, there has existed a need for an accurate, reliable andcost-efficient spray cooling system. The present invention satisfiesthese and other needs, and provides further related advantages.

SUMMARY OF THE INVENTION

The present invention provides a spray cooling system for cooling a heatsource, embodiments of which can exhibit improved accuracy, reliabilityand/or cost efficiency. Embodiments of the invention typically featurean incremental sprayer configured to eject an incremental amount of thecooling fluid on the heat source. The cooling fluid is sprayed inresponse to a control signal, which is sent to the sprayer by acontroller.

Advantageously, these features provide for accurate delivery of coolingfluid at precise and controllable rates. The technology for this type ofincremental sprayer is well developed in the ink-jet printer arts, andit is relatively inexpensive to manufacture. Furthermore, the design canbe modular, offering quickly and easily replaceable units.

The invention further features the use of thermal ink-jet technology indesigning the sprayer. In particular, the embodiment of the inventionmay have a body defining a chamber configured to hold a volume of thecooling fluid, and defining an orifice in communication with thechamber. A heating element is in thermal communication with the chamber,and is configured to vaporize a portion of the cooling fluid held withinthe chamber. The orifice is configured to direct cooling fluid from thechamber to the heat source upon the heating element vaporizing a portionof the cooling fluid held within the chamber.

This technology generally provides for efficient delivery of the coolingfluid to the heat source. Some known inert cooling fluids haveviscosities and boiling points similar to that of ink-jet ink, and theink-jet sprayers are typically adaptable to use with the cooling fluids.Furthermore, unlike typical ink-jet ink, cooling fluid does not containparticulate matter that can clog the system. Thus, the system is bothreliable and cost efficient to design.

The invention further features the ejection of incremental amounts of acooling fluid on the heat source, using an incremental sprayer, spacedover a number of time increments. Either the incremental time or theamount ejected can be varied to adjust the flow rate to an optimallevel. The system can be controlled by monitoring, either directly orindirectly, the temperature of the heat source and the amount of poolingor dry-out that is occurring, if any. This can provide for optimizedcooling of a heat source.

Other features and advantages of the invention will become apparent fromthe following detailed description of the preferred embodiments, takenin conjunction with the accompanying drawings, which illustrate, by wayof example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a cooling system embodyingfeatures of the present invention.

FIG. 2 is a cross-sectional view of a sprayer for the cooling systemrepresented in FIG. 1.

FIG. 3 is a cut-away perspective view of a first embodiment of thecooling system represented in FIG. 1.

FIG. 4 is a cross-sectional view of the embodiment depicted in FIG. 3.

FIG. 5 is a cut-away perspective view of a second embodiment of thecooling system represented in FIG. 1.

FIG. 6 is a cross-sectional view of a third embodiment of the coolingsystem represented in FIG. 1.

FIG. 7 is a control system block diagram for controlling the operationof the embodiment depicted in FIG. 6.

FIG. 8 is a control system block diagram for controlling the operationof the embodiment depicted in FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of a cooling assembly 10 for cooling a heat-generatingsemiconductor device 12, according to the present invention, isschematically shown in FIG. 1. The assembly includes a one or moreincremental sprayers 14 for spraying an incremental amount of a liquidcooling fluid 16, preferably from a reservoir 18, onto the semiconductordevice to evaporatively cool the semiconductor device. The assembly alsoincludes a heat exchanger 20 to extract the heat from the vaporizedcooling fluid, and thereby liquify or condense it. The assembly furtherincludes a pump 22 to pump the liquified cooling fluid back into thereservoir that feeds the sprayers.

While incremental sprayers 14 that can be used as part of the presentinvention can be based on other types of ink-jet droplet expellingtechnology, such as piezoelectric technology (i.e., piezoelectricnozzles), they are preferably based on thermal ink-jet technology.Examples of this technology are discussed in numerous U.S. Patents,including U.S. Pat. Nos. 5,924,198, 4,500,895, and 4,683,481, which areincorporated herein by reference. Other thermal ink-jet technologies canlikewise be appropriate for use with this invention. A highly preferablecooling fluid for use with a thermal incremental sprayer is 3MFluorinert®, which is easily adaptable to existing thermal ink-jettechnology because it has a viscosity and boiling point similar to thatof the inks typically used in ink-jet printers.

With reference to FIG. 2, which depicts two simplified, exemplaryincremental sprayers 14, each sprayer includes structure defining achamber 30 for receiving a predetermined portion of the cooling fluidand a heater 32 for vaporizing a portion of the cooling fluid, to createthe pressure to eject an incremental amount of the cooling fluid throughan orifice 34 that directs the ejected cooling fluid toward thesemiconductor device 12 (FIG. 1). The orifices are formed in a flexiblepolymer tape 36, e.g., tape commercially available as Kapton TM tape,from 3M Corporation.

Affixed to a back surface 38 of the tape 36 is a silicon substrate 40containing the heaters 32, in the form of individually energizablethin-film resistors. Each heater is preferably located on a side of thechamber 30 across from the chamber's orifice 34. Cooling fluid ispreferably drawn and loaded into the chamber by capillary action, as istypical for an ink-jet type device. A computerized controller (notshown) energizes the heater, vaporizing the portion of the cooling fluidadjacent to the heater. The vaporized cooling fluid expands, expellingmost of the non-vaporized cooling fluid out of the orifice, typically inthe form of a single droplet.

Depending on the configuration of the sprayer, the incremental amount ofthe fluid sprayed from the sprayer could be in the form of a singledroplet, or in the form of multiple droplets. Multiple droplets could beproduced by multiple orifices related to a single heater, or by sprayershaving larger chamber volumes and appropriately shaped orifice nozzlesto cause the incremental amount of fluid to break into droplets. Afterthe chamber has been fired by the heater, capillary action again loadsthe chamber for a subsequent firing.

The liquid spray from the incremental sprayers 14 can be highlycontrollable. For example, by increasing or decreasing the frequencythat the sprayers are energized, the flow rate can be accuratelyadjusted. Furthermore, because the sprayers can be configured to deliververy small quantities of cooling fluid, and because a large number ofsprayers can be fit into a small area, the heat distribution over thatarea can be accurately controlled by energizing some of the sprayers ata rate greater than that of other sprayers.

With reference again to FIG. 1, to aid the reservoir 18 in providing thecooling fluid to the incremental sprayers 14, the reservoir can beconfigured with a spring assist mechanism 42. Alternatively, thereservoir can be positioned such that the cooling fluid receives agravity assist in flowing to the sprayers 14. In addition to serving asa source of liquid cooling fluid, the reservoir also serves to separateany gas leaving the condenser. A filter (not shown) can be used, eitherin the reservoir or in some other portion of the system, to remove boardlevel contaminants that are present in the system.

The pump 22 serves to replenish the reservoir 18, and can be a low-costapparatus that does not provide either high pressure or consistent andcontrolled flow. Preferably the pump should be self priming to removetrapped gas.

The precise order of the components can be varied. For example, the pump22 could be placed prior to the heat exchanger 20, so long as it canpump both vapors and fluids. Likewise, depending on the type of sprayer,the reservoir could be eliminated, and the pump could be used todirectly feed the sprayers 14. The entire assembly 10, including thecircuit board, is preferably a field-replaceable unit

With reference now to FIGS. 3 and 4, multiple cooling systems within onecomputer (or other device) can be configured to share components. Thecomputer can contain a plurality of circuit boards 50 carryingheat-generating components 52 such as CPUs, each circuit board typicallybeing mounted on a backplane 54. Incremental sprayers 56 are locatedadjacent to the components, and are configured to spray the componentswith a cooling fluid. The components and sprayers are enclosed in acompartment 58 that prevents vaporized cooling fluid from escaping thesystem. A roll bond panel 60 serves as a first heat exchanger,condensing some or all of the vapor. The roll bond panel is formed as awall of the sealed compartment. Suitable roll bond panels can beobtained from Showa Aluminum Corporation, of Tokyo, Japan, or fromAlgoods, of Toronto, Canada. A suitable low-boiling point working fluid,e.g., 3M Fluorinert®, is carried within fluid channels in the roll bondpanel. Alternatively, working fluids such as hydrofluoroether or alcoholcould be used.

A second heat exchanger 62, which can also be a roll bond panel, islocated externally from the circuit board compartments 58, and providesfor the additional condensing of vaporized cooling fluid. The secondheat exchanger receives the cooling fluid, which can be both liquid andvaporized, from the compartments of each circuit board 50. After thecooling fluid has been further cooled by the second heat exchanger, acommonly shared pump 64 delivers the cooling fluid to a commonly sharedreservoir 66, which in turn returns the cooling fluid to the sprayers 56of each circuit board.

With reference now to FIG. 5, the entire cooling system can beincorporated into a single circuit board assembly 70. The circuit boardassembly will typically include heat-generating components 72 such asCPUs, mounted on a circuit board 74. Incremental sprayers 76 are locatedadjacent to the components, and are configured to spray the componentswith a cooling fluid. The components and sprayers are enclosed in asealed compartment 78 that prevents vaporized cooling fluid fromescaping. One or more roll bond panels 80 preferably are incorporatedinto one or more compartment walls, and are configured to condense vaporand release it into a collection reservoir 82 in the bottom of thecircuit board assembly. The pool also receives non-vaporized coolantthat drips from the components. A pump 84 pumps the cooling fluid upinto a main feed reservoir 86, preferably being above the sprayers,which provides the cooling fluid to the sprayers. As an alternative tothe reservoir's being located above the sprayers, which causes a gravityfeed effect, the reservoir could incorporate some type of pressuremechanism, such as a spring.

Generally speaking, for embodiments of the invention to function atoptimal efficiency, the sprayers' mass flow rate ({dot over (m)}_(s))should be adjusted to avoid having the semiconductor device becomeeither dry or immersed. This rate is controlled by having a controlleradjust the rate that the thermal jets are fired. The optimum mass flowrate can change as the heat flux of the semiconductor device changes.Thus, for a controller to correctly control the mass flow, parameters ofthe semiconductor device and/or cooling system need to be sensed.

To determine whether the mass flow rate is at an optimal level, sensorscan be used to track one or more of the system parameters. The types ofparameters that are available vary with the type of system employed. Forexample, if the heat exchanger is external to the chamber where thespraying occurs, then the liquid and the vapor can be removed from thechamber through separate passages (with the assistance of a resistivemesh to inhibit entry of vapor into the liquid passage), and the massflow of the vapor ({dot over (m)}_(v)) and/or mass flow of the liquid({dot over (m)}_(l)) are available to be measured. However, these arenot available if the heat exchanger is within the spray chamber, such asin the embodiment of FIG. 5. Instead, the vapor pressure within thespray chamber (P_(v)) and the semiconductor device's junctiontemperature can be sensed.

FIG. 6 depicts a cooling system 90 having a heat exchanger 92 externalto a spray chamber 94. The spray chamber contains incremental sprayers96 that spray a cooling fluid onto semiconductor devices 98 on a circuitboard 100. Depending on the temperature of the semiconductor devices,some of the cooling fluid may vaporize, and some may run off to form apool 102. Vapor exits the spray chamber through a vapor passage 104,while liquid exits via a liquid passage 106. A mesh 108 is used toprevent vapor from entering the liquid passage, while gravity preventsthe liquid from entering the vapor passage. The vapor and liquid arecombined and inserted into the heat exchanger 92, which removes heat andliquefies the vapor. A pump 110 draws the cooled cooling fluid up into areservoir 112, where it is again provided to the sprayers.

A number of potentially useful system parameters can be sensed in thissystem, including: The temperature of the semiconductor devices (T_(j))(i.e., the junction temperature), which can often be sensed from withinthe device; The ambient temperature (T_(a)) and pressure (P_(a)), aswell as the vapor pressure (P_(v)), in the spray chamber 94, which canbe sensed using temperature and pressure sensors 114 within the spraycompartment; The mass flow of the vapor ({dot over (m)}_(v)) and themass flow of the liquid ({dot over (m)}_(l)), which can be sensed usingappropriate sensors 116, 118 in respective vapor and liquid passages120, 122; The temperature (T_(so) of the sub-cooled liquid coming out ofthe heat pump 92, which can be sensed by a temperature sensor; And thetemperature (T_(s)) of the liquid being received by the sprayer.

With reference to FIG. 7, a method of adjusting the sprayers' mass flowrate ({dot over (m)}_(s)) for the device depicted in FIG. 6 begins withthe steps of by startng the cooling system 120 and setting 122 thesprayers' initial mass flow rate at an initial value ({dot over(m)}_(s,init)). This value typically would be based on prior experiencewith this system, or with systems of its type, but could also be basedon calculated heat generations rates and cooling rates. A limited amountof time (t) is preferably allowed to pass 124 so that the system canbegin functioning, and then the sensing logic begins to take action,i.e., the cooling system begins sensing and monitoring parameters andadjusting the sensors' mass flow rate.

In particular, the temperature of the semiconductor devices (T_(j)) issensed 126, and the resulting sensor value is compared 128 to a selectedmaximum value T_(max). If the resulting sensor value is below theselected maximum value then no action is taken, and the monitoring ofparameters is repeated. If, however, the semiconductor device hasreached the selected maximum value, then sensors are used to determineif pooling is occurring. Preferably, to detect pooling, the mass flow ofthe vapor ({dot over (m)}_(v)) is sensed 130 and compared 132 to aselected minimum value ({dot over (m)}_(v,min)) to verify that it isabove that value {dot over (m)}_(v,min). The selected minimum value({dot over (m)}_(v,min)) typically would be based on prior experiencewith this system, or with systems of its type, but could also be basedon calculated heat generations rates and cooling rates.

If the mass flow of the vapor ({dot over (m)}_(v)) is above the selectedminimum value, then pooling is not a problem, and the sprayers' massflow rate should be increased 134 to reduce the temperature (T_(j)).However, if the mass flow rate of the vapor is not above the minimumvalue, then pooling is occurring and the sprayers' mass flow rate isdecreased 136 to increase the cooling system's effectiveness. After thesprayers' mass flow rate is incrementally adjusted, either up or down,the monitoring is continued by again sensing the temperature of thesemiconductor devices (T_(j)). It should be noted that the orientationof the sprayed surface (with respect to gravity) might have an effect onthe accurate sensing of pooling, and that experimentation can be used toverify and/or adjust the selected minimum value {dot over (m)}_(v,min)accordingly.

In the alternative, other sensors can be used to determine if pooling isoccurring. For example, the vapor pressure (P_(v)) in the spray chamberis a more direct measure of whether pooling is occurring.

FIG. 8 is a flowchart depicting a method of adjusting the sprayers' massflow rate ({dot over (m)}_(s)) for a cooling device having an internalheat exchanger, such as the device depicted in FIG. 5. The method beginswith the steps of starting the cooling system 140 and setting thesprayers' initial mass flow rate 142 at an initial value ({dot over(m)}_(s,init)). This value typically would be based on experimentation,and/or prior experience with this system or systems of its type, but itcould be based on an analysis of temperature generation rates andcooling rates. A limited amount of time (t) is preferably allowed topass 144 prior to starting the actions of the sensing logic, so that thesystem can begin functioning, and the cooling system can begin sensingand monitoring parameters and adjusting the sensors' mass flow rate. Thetime (t) is related to the time constant of the system, i.e., the timeneeded for the system to reach operating temperatures.

In particular, the temperature of the semiconductor devices (T_(j)) issensed 146, and the resulting sensor value is compared 148 to a selectedmaximum value T_(max). If the resulting sensor value is below theselected maximum value then no action is taken, and the monitoring ofparameters is repeated. If, however, the semiconductor device hasreached the selected maximum value, then sensors are used to determineif pooling is occurring. Preferably, to detect pooling, the vaporpressure (P_(v)) in the spray chamber is sensed 150 and compared 152 toa selected minimum value (P_(v,min)) to verify that it is above theselected minimum value. To aid in the accurate sensing of the vaporpressure, the system is preferably configured with an internal pressurebelow atmospheric pressure. The selected minimum value (P_(v,min)) isnot easy to calculate, and is preferably determined empirically.

If the vapor pressure (P_(v)) in the spray chamber is above the selectedminimum value, then pooling is not a problem, and the sprayers' massflow rate should be increased 154 to reduce the temperature (T_(j)).However, if the vapor pressure (P_(v)) is not above the minimum value,then pooling is occurring and the sprayers' mass flow rate is decreased156 to increase the cooling system's effectiveness. After the sprayers'mass flow rate is incrementally adjusted, either up or down, themonitoring is continued by again sensing the temperature of thesemiconductor devices (T_(j)).

More generally, it will be seen that any sensor reading indicative ofthe semiconductor's temperature, including direct readings or indirectreadings (such as heat dissipation when the heat generation rate isknown) can be used to judge whether the cooling is adequate.Furthermore, it will be seen that any sensor reading indicative ofpooling, such as vapor flow rate, liquid flow rate, vapor pressure, orothers, can be used to judge whether the cooling would be improved byincreasing or decreasing the spray flow rate. Additionally, it will beappreciated that, while the order of sensing and decision makingcontributes to the efficiency of the system, it can be varied within thescope of the invention. For example, both temperature and pooling can besensed prior to any comparisons. Likewise, pooling can be sensed andcompared to a reference value prior to sensing the semiconductor (orother heat-generating device) temperature.

From the foregoing description, it will be appreciated that the presentinvention provides an accurate, reliable and cost efficient spraycooling system. The system includes a sprayer configured to delivercooling fluid to a heat-generating device in limited increments.Preferably, the sprayer is thermally driven in a fashion similar to thatof an ink-jet print head.

While a particular form of the invention has been illustrated anddescribed, it will be apparent that various modifications can be madewithout departing from the spirit and scope of the invention. Thus,although the invention has been described in detail with reference onlyto the preferred embodiments, those having ordinary skill in the artwill appreciate that various modifications can be made without departingfrom the invention. Accordingly, the invention is not intended to belimited, and is defined with reference to the following claims.

We Claim:
 1. A cooling assembly for cooling a semiconductor with acooling fluid, comprising: an incremental sprayer configured to eject anincremental amount of the cooling fluid on the semiconductor in responseto a control signal; and a controller configured to send the controlsignal to the incremental sprayer.
 2. The cooling assembly of claim 1,wherein the incremental sprayer comprises: a body defining a chamberconfigured to hold a volume of the cooling fluid, and defining anorifice in communication with the chamber; and a heating element inthermal communication with the chamber, the heating element beingconfigured to vaporize a portion of the cooling fluid held within thechamber; wherein the orifice is configured to direct cooling fluid fromthe chamber to the heat source upon the heating element vaporizing aportion of the cooling fluid held within the chamber.
 3. The coolingassembly of claim 2, wherein: the body includes a thin-film substrateand a backing; the chambers are cavities that are formed adjoining oneside of the thin-film substrate by the backing; the orifice is a passagethrough the thin-film substrate; and the heating element is a thin-filmresister.
 4. The cooling assembly of claim 1, wherein the incrementalamount of the cooling fluid is a single droplet of cooling fluid.
 5. Thecooling assembly of claim 1, wherein the incremental sprayer comprises apiezoelectric nozzle.
 6. The cooling assembly of claim 1, and furthercomprising a heat exchanger configured to cool and condense coolingfluid that was vaporized by the heat source after being ejected by theincremental sprayer.
 7. The cooling assembly of claim 6, wherein theheat exchanger is a roll bond panel.
 8. The cooling assembly of claim 1,and further comprising a reservoir configured to provide liquid coolingfluid to the incremental sprayer.
 9. The cooling assembly of claim 1,and further comprising: a heat exchanger configured to cool and condensecooling fluid that was vaporized by the heat source after being ejectedby the incremental sprayer; a reservoir configured to provide liquidcooling fluid to the incremental sprayer; and a pump configured to pumpliquid cooling fluid from the heat exchanger to the reservoir.
 10. Acooled circuit board, comprising: a circuit board; a semiconductordevice mounted on the circuit board; and a cooling assembly configuredto cool the semiconductor device with a cooling fluid, wherein thecooling assembly includes an incremental sprayer configured to eject anincremental amount of the cooling fluid on the semiconductor device inresponse to a control signal and wherein the cooling assembly includes acontroller configured to send the control signal to the incrementalsprayer.
 11. The cooled circuit board of claim 10, and furthercomprising: a heat exchanger configured to cool and condense coolingfluid that was vaporized by the semiconductor device after being ejectedby the incremental sprayer; a reservoir configured to provide liquidcooling fluid to the incremental sprayer; and a pump configured to pumpliquid cooling fluid from the heat exchanger to the reservoir.
 12. Acooled circuit board assembly, comprising: a plurality of cooled circuitboards, each circuit board comprising: a circuit board; a semiconductordevice mounted on the circuit board; and a cooling assembly configuredto cool the semiconductor device with a cooling fluid, wherein thecooling assembly includes an incremental sprayer configured to eject anincremental amount of the cooling fluid on the semiconductor device inresponse to a control signal, and wherein the cooling assembly includesa controller configured to send the control signal to the incrementalsprayer; a heat exchanger configured to cool and condense cooling fluidthat was vaporized by the semiconductor device of each of the pluralityof cooled circuit boards after being ejected by the incremental sprayer;a reservoir configured to provide liquid cooling fluid to theincremental sprayer of each of the plurality of cooled circuit boards;and a pump configured to pump liquid cooling fluid from the heatexchanger to the reservoir.
 13. The cooled circuit board assembly ofclaim 12, wherein each of the plurality of cooled circuit boardsincludes a roll bond panel configured to cool and condense cooling fluidthat was vaporized by the semiconductor device after being ejected bythe incremental sprayer.
 14. A method of cooling a semiconductorcomprising: ejecting an incremental amount of a cooling fluid onto thesemiconductor using an incremental sprayer; and repeating the step ofejecting at time increments.
 15. The method of claim 14, wherein thestep of spraying comprises: providing cooling fluid to a chamber definedin the incremental sprayer; energizing a heating element in thermalcommunication with the chamber to vaporize a portion of the coolingfluid within the chamber; wherein the incremental sprayer defines anorifice in communication with the chamber, the orifice being configuredto direct cooling fluid from the chamber to the heat source upon theheating element's vaporizing of a portion of the cooling fluid in thechamber.
 16. The method of claim 15, wherein: in the step of providing,the chamber is a cavity formed in a backing, the cavity adjoining athin-film substrate; the orifice is a passage through the tin-filmsubstrate; and in the step of heating, the heating element is athin-film resister.
 17. The method of claim 14, and further comprising:sensing the temperature of the heat source; sensing the pooling of thesprayed cooling fluid; adjusting the mean flow rate of cooling fluidejected by the incremental sprayer based on the sensed temperature ofthe heat source and pooling of the sprayed cooling fluid.
 18. The methodof claim 17, wherein the mean flow rate is adjusted by adjusting thefrequency with which the step of ejecting is repeated.
 19. The method ofclaim 14, wherein the step of repeating is conducted with a frequencythat varies with a sensed indication of pooling of the sprayed coolingfluid.
 20. The method of claim 14, and further comprising: sensing thetemperature of the heat source; sensing whether pooling is occurring;and varying the frequency with which the step of repeating is conductedbased upon the sensed temperature and the sensed result of whetherpooling is occuring; wherein if the sensed temperature is above apreviously determined maximum temperature and the sensed result ofwhether pooling is occurring indicates that pooling is occurring, thefrequency with which the step of repeating is conducted is varieddownward in the step of varying; and wherein if the sensed temperatureis above a previously determined maximum temperature and the sensedresult of whether pooling is occurring indicates that pooling is notoccurring, the frequency with which the step of repeating is conductedis varied upward in the step of varying.